In optical disks such as a magneto-optical disk and a phase change disk, lands and grooves are formed alternately in a radial direction, and signals are recorded on both the lands and the grooves for achieving a high density.
According to AS-MO (Advanced Storaged Magneto Optical disk) standards, which are recently established, fine clock marks forming a reference for producing a clock, which is used for recording or reproducing data, are formed at predetermined intervals. The fine clock marks are specifically formed in such a manner that grooves each having a length of about 3 to 4 data channel bits are formed at predetermined intervals on the land, and lands each having a length of about 3 to 4 data channel bits are formed at predetermined intervals in the groove. The fine clock marks in the groove may be formed on one of surfaces of a substrate of a magneto-optical record medium by using two beams, in which case the two beams are moved to positions of the neighboring lands on opposite sides of the groove. The fine clock marks on the land are formed similarly to the fine clock marks in the groove by moving two beams to positions in the neighboring grooves on the opposite sides.
In the magneto-optical disk according to the AS-MO standards, a fine clock mark detection signal is produced by detecting the fine clock marks on the magneto-optical disk. A clock is produced based on the fine clock mark detection signal, and recording and reproducing of signals are performed in synchronization with the clock thus produced.
A scratch may be formed between fine clock marks, for example, during the manufacturing process of the magneto-optical disk, and a fine clock mark signal detected from the magneto-optical disk may contain a signal component due to the scratch. In this case, the clock cannot be produced accurately.
More specifically, if a scratch is present between fine clock marks, the fine clock mark signal has a signal waveform shown in FIG. 19. Signal components S1, S3 and S4 are generated based on fine clock marks, respectively, and a signal component S2 is generated due to the scratch. When the groove is scanned with a laser beam, signal components S1, S3 and S4 exhibit signal waveforms opposite to those exhibited when the land is scanned with the laser beam. For accurately producing the clock, therefore, it is necessary to determine whether the scanning with the laser beam is effected on the groove or the land. For this, signal components S1, S3 and S4 are compared based on levels L1 and L2, and it is determined whether the scanning with the laser beam is effected on the groove or land, depending on the level (i.e., level L1 or L2) used for earlier detection of the signal. Level L1 is determined to be substantially equal to half a peak value PA of signal component S1, which is held by a hold circuit. Level L2 is determined to be substantially equal to half a bottom value PB of signal component S1, which is held by the hold circuit. The hold circuit substantially follows peak value PA and bottom value PB even when a long time elapses from holding of peak and bottom values PA and PB of signal component S1.
If signal component S2 due to the scratch is applied to the hold circuit between input of signal component S1 and input of signal component S3, level L1 determined from the peak value shifts to level L3, and level L2 determined from the bottom value shifts to level L4 because the peak and bottom values of signal component S2 are larger than the peak and bottom values of each of signal components S1, S3 and S4 based on the fine clock marks, respectively. Since the hold circuit follows the peak or bottom values, level L3 is held at a value larger than the peak values of signal components S3 and S4, and level L4 is held at a value smaller than the bottom values of signal components S3 and S4. If signal component S2 due to the scratch is present, therefore, it is impossible to compare the subsequent signal components based on the fine clock marks, and the clock cannot be produced accurately.